Example Data Table
| Input |
Example value |
Meaning |
| Floor area |
75 m2 |
Conditioned room area |
| Wall U value |
0.70 W/m2K |
Wall heat transfer rate |
| Outdoor and indoor temperature |
38 C and 24 C |
Cooling temperature difference |
| Ventilation |
400 CFM |
Fresh air or leakage flow |
| People |
8 occupants |
Internal sensible and latent heat |
| Safety factor |
10% |
Allowance for uncertainty |
Formula Used
Temperature difference: DT = outdoor temperature - indoor temperature
Transmission load: Q = U x A x DT
Solar glass load: Q = window area x solar gain x shading factor
People load: Q = people x load per person
Lighting load: Q = floor area x lighting density
Equipment load: Q = equipment watts x duty factor
Ventilation sensible load: Q = air density x air specific heat x airflow x DT
Ventilation latent load: Q = air density x airflow x humidity ratio difference x latent heat
Total load: total = sensible load + latent load + safety allowance
Cooling tons: tons = BTU per hour / 12000
How to Use This Calculator
Enter the room size, surface areas, U values, temperatures, humidity values, ventilation rate, occupancy, lighting, and equipment data.
Use CFM, L/s, m3/h, or ACH for the ventilation field. Select the matching unit before calculating.
Press the calculate button to show results above the form. Review the breakdown to find the largest heat source.
Use the CSV button for spreadsheet work. Use the PDF button for a simple report copy.
About heat load calculation
Heat load calculation estimates the cooling capacity required for a room or building. It studies heat that enters through walls, roofs, glass, fresh air, people, lights, and machines. A careful estimate helps prevent undersized cooling systems. It also avoids oversizing, which can increase cost, cycling, humidity issues, and noise.
Why the method matters
This calculator uses a practical engineering approach for early design. It separates the load into transmission, solar, internal, ventilation, and latent parts. Transmission covers heat flow through surfaces. Solar load covers sunlight through glass. Internal load covers people, lighting, and equipment. Ventilation load covers outdoor air brought into the room. Latent load covers moisture that must be removed.
Inputs that affect results
Area, temperature difference, U values, air changes, glass exposure, and occupancy have strong influence. A room with large windows can need more cooling than a bigger shaded room. High outdoor humidity can also increase total capacity, even when the sensible load seems moderate. Equipment duty factor is useful because many devices do not run at full power all day.
Using the output
The result shows watts, kilowatts, BTU per hour, and cooling tons. The breakdown helps you find the largest heat source. If the window or roof load is high, shading or insulation may reduce capacity. If ventilation load is high, heat recovery or demand control may help. The suggested airflow is based on sensible heat and supply temperature difference.
Design note
This tool is best for planning, comparison, teaching, and quick checks. Final HVAC sizing should include local codes, detailed weather data, orientation, schedules, duct losses, and professional judgement. Always review assumptions before buying equipment, because comfort depends on humidity control, airflow pattern, filtration, and installation quality.
Reading the breakdown
The breakdown is often more useful than the total alone. Large wall or roof gains point to insulation problems. Large glass gains suggest shading, lower solar heat gain glass, or reduced exposed area. Large latent gains suggest humidity control or lower outside air leakage. A balanced design improves comfort and operating cost.
Accuracy tips
Use measured dimensions. Enter realistic peak outdoor temperature and humidity. Select U values from product data. Replace guesses after drawings, schedules, and equipment lists become available.
FAQs
What is heat load?
Heat load is the cooling capacity needed to remove heat from a space. It includes heat from walls, roof, windows, people, lighting, equipment, ventilation, and moisture.
What is sensible heat load?
Sensible heat changes air temperature. It comes from conduction, sunlight, lights, equipment, people, and warm outdoor air. It is used for airflow sizing.
What is latent heat load?
Latent heat is moisture removal load. It does not directly show as temperature change, but it affects humidity, comfort, and cooling equipment capacity.
Can this replace professional HVAC design?
No. It is useful for estimates, comparisons, and learning. Final design should check codes, climate data, orientation, duct losses, schedules, and equipment selection.
What is a U value?
A U value shows how easily heat passes through a surface. Lower U values mean better insulation and lower transmission load.
Why does ventilation increase load?
Ventilation brings outdoor air into the room. That air may be hotter and more humid, so the cooling system must remove extra heat and moisture.
What does cooling tons mean?
Cooling tons are a common cooling capacity unit. One cooling ton equals 12,000 BTU per hour.
Why add a safety factor?
A safety factor covers uncertainty in inputs, weather, occupancy, leakage, and operating conditions. Use it carefully because too much safety can oversize equipment.